This invention relates to correcting video color or neutral signals and, more particularly, to a method and apparatus for selectively producing video color correction signals or color corrected resultant signals having a uniform hue, saturation or luminance.
In video color recording, the images may be represented by either of two sets of signals. One signal set is red/green/blue (RGB). The other signal set is luminance (Y) and color difference signals, i.e. red minus luminance (R-Y) and blue minus luminance (B-Y).
In the course of production of television and video color recordings, it is sometimes desirable to correct selectively the colors of the subject images. Many years ago Monahan et al. disclosed a non-linear matrixing device for color correction in U.S. Pat. No. 3,558,806. This matrixing device operates upon the red/green/blue signals to permit independent adjustment of the hue and saturation of the three primary colors, red, green, and blue, and the secondary colors, cyan, yellow, and magenta. When an object made up of more than one color is to be color corrected, a coordinated adjustment of both colors is required.
Recently, Korman disclosed in U.S. Pat. No. 4,525,736 color modification apparatus which operates upon the color difference signals known as I and Q, rather than the primary and secondary color signals. The color signals to be corrected are selected by limit circuits which determine when the difference signals lie between prescribed boundaries in the chrominance plane. These boundaries define rectangular regions in the chrominance plane which do not correspond to the polar coordinate color corrections actually made by the colorist. As a result, the colorist may encounter difficulty in selecting the signals to be color corrected.
Kaye, PCT Application No. PCT/US87/02240, Publication Date Mar. 24, 1988, describes a video signal color correction apparatus which selects for color correction areas of the chrominance plane defining sectors in polar coordinates. Both the angle and the width of the sector can be changed to select precisely the desired signals for color correction. Specifically, component color video signals which only lie within a fixed hue sector are transmitted from an input to an output. Responsive to a first control signal, the difference signals are rotated through an angle which corresponds to a selected hue sector relative to the fixed hue sector in which the component color signals are to be corrected. The rotated difference signals are applied to the input to produce at the output an isolated color signal only when the difference signals lie within the selected hue sector. The width of the fixed hue sector is reduced responsive to a second control signal. The first and second control signals permit selection of the angle and the width of the sector in which the component signals are to be corrected.
In many situations, ordinary color signals include signals representing hues adjacent to one another but which have varied saturations. In a typical scene, the adjacent hues may represent the hues on a person's face or hues in a background, etc., wherein the adjacent hues do not have all the same saturation. In the Kaye specification, the hues found in a given hue sector can be isolated and added to or subtracted from the uncorrected signals in order to modify or entirely eliminate the corresponding hues. Where the original hues are only modified, saturations may be increased or decreased typically an amount in proportion to the relative saturations of the original signals. However, it is sometimes desirable to modify the saturations of the isolated hues such that the differences between the saturations of adjacent hues remains approximately the same. Additionally, there are instances where it is desired to have the saturations of all of the isolated hue signals substantially equal.
There is a need for a color correction system which can produce color correction signals where the saturations of hue signals to be corrected are modified the same amount for all isolated hues, which can produce hue signals in a color correction signal having equal saturation levels, or which can produce one color correction signal having the isolated hues with equal saturations at a first level and a second color correction signal having the same isolated hues with equal saturations at a second level.
There is also a need for a color correction system which can take the "white" signal portions of the color video and apply them to a plurality of correction channels to modify isolated grey levels without affecting other grey levels. There is also a need for a color correction system which can take the luminance, or Y, signals in the color video set of signals and apply them to a plurality of correction channels to modify either or all of the uncorrected luminance signals, the uncorrected hue signals or the saturation levels of those hue signals. In other words there is a need for a color correction system having a plurality of correction channels which can accept isolated signals, white signals or luminance signals in order to modify the hue, saturation, luminance or grey levels in a video picture. The disclosed system takes advantage of each of these possible sources of video signals for isolation and subsequent use as correction signals.